A Novel Selective Cleavage Method for β- O -4 Substructure in Lignins Named TIZ Method. I. Degradation of Guaiacyl and Syringyl Models

Abstract

A novel degradation method of arylglycerol-β-aryl (β-O-4) ether substructure in lignins without any secondary condensation reactions named TIZ method was developed. The TIZ method consists of three reaction steps: (1) selective tosylation (T) of primary hydroxyl group, (2) iodination (I), and (3) zinc-metal treatment (Z) to cleave of β-O-4 linkage. This method was applied to two nonphenolic β-O-4 lignin substructure compounds of guaiacyl and syringyl type: benzylguaiacylglycerol-β-guaiacyl ether (1a) and benzylsyringylglycerol-β-syringyl ether (1b). Respective reaction steps were performed quantitatively concerning both guaiacyl and syringyl models. After zinc treatment, almost 1 mole of olefinic compounds, 3-benzylguaiacyl-3-hydroxy-1-propene (4a) and 3-benzylsyringyl-3-hydroxy-1-propene (4b), were formed from 1 mol of compound 1a and 1b, respectively. This indicates that β-O-4 linkage in compound 1a and 1b was cleaved quantitatively without formation of any by-products. The structures of the degradation products from compound 1a and 1b by TIZ method were determined by 1H and 13C NMR spectroscopy and GC–MS, and by comparison with authentic compounds synthesized independently. The degradation products were quantified by GC. The TIZ method, carried out under weak alkaline and neutral reaction conditions, was shown to be useful for the selective cleavage of β-O-4 linkage of compound 1a and 1b without secondary condensation reaction on side chain. Therefore, this method is an efficient analytical method for lignin structure studies.